Position-measuring devices may be used to determine the position of two objects movable relative to each other, and during operation, generate (electrical) output signals which are supplied (in digital form) as measured position values (position data) to a control and processing unit, in which the measured position values are evaluated and used, for example, to control a machine tool. Conversely, the control and processing unit also transmits data regularly to the position-measuring device, for instance, in the form of position-request instructions which trigger the transmission of measured position values from the position-measuring device to the control and processing unit, or in the form of parameter data, which is important for the operation of the position-measuring device. Thus, a position-measuring device for determining the position of two objects movable relative to each other may be connected via a data-transmission channel to a control and processing unit.
The data-transmission channel, which produces a physical connection between the position-measuring device and the control and processing unit and thereby permits data to be transmitted between those two modules, together with the rules of the data transmission denoted as a protocol, is also referred to as interface.
Various interfaces may be used for transmitting data between a position-measuring device and an associated control and processing unit. The type of interface used may depend, first of all, on whether the position-measuring device is an incremental position-measuring device with which changes in position between two objects movable relative to each other are determined, from which absolute positions may possibly also be determined by referring to a reference position (which is ascertainable in an initial reference execution of the position-measuring device), or whether it is an absolute position-measuring device with which the position of one of the two objects relative to the other of the two objects is ascertainable directly. Moreover, even in the case of a selected type of a position-measuring device (absolute or incremental), however, different interfaces are used for the connection to a control and processing unit.
For example, serial interfaces are used for the most part for transmitting absolute measured position values from a position-measuring device to a control and processing unit, since they are able to achieve high data-transmission rates using only a few data-transmission lines. This holds true especially for what are called synchronous serial interfaces, which have a unidirectional or bidirectional data line and a clock line. In that case, data packets are transmitted on the data line in synchronism with a clock signal on the clock line. Examples for such synchronous serial interfaces are referred to as the EnDat interface and the SSI.
The SSI interface is described, for example, in European Patent Application No. 0 171 579. It is in the form of a synchronous serial interface having a unidirectional data line and a unidirectional clock line. Measured position values are read out from a position-measuring device in synchronism with a clock signal on the clock line.
The fundamentals of the EnDat interface are explained in European Published Patent Application No. 0 660 209. It is likewise a synchronous serial interface which, however, besides a unidirectional clock line, has a bidirectional data line. Data is thereby able to be transmitted in both directions, e.g., from the position-measuring device to the control and processing unit, and conversely, from the control and processing unit to the position-measuring device. The data is transmitted in synchronism with a clock signal on the clock line, as well.
When using the two aforesaid interfaces, the position-measuring device is connected in each case to the control and processing unit via a data-transmission channel which is formed by two pairs of lines. In contrast, German Published Patent Application No. 10 2008 027 902 describes an interface version where the position-measuring device is connected to the control and processing unit via only one pair of lines.
Accordingly, to form a data-transmission channel, a position-measuring device is connectable via a first pair of lines, or alternatively, via a first and at least one further (second) pair of lines to a control and processing unit, in order to be able to transmit data.
The control and processing unit is also referred to herein as a processing unit, because the reception and processing of data, obtained from the position-measuring device and transmitted via a data-transmission channel (interface), are among important functions of that unit. Moreover, it may also be used to control the position-measuring device on one hand, and/or, on the other hand, a machine tool or other assembly which is to be controlled as a function of the data obtained with the aid of the position-measuring device.
German Published Patent Application No. 10 2008 054 887 describes a device and a method for the automated differentiation of the two first-named interfaces, each of which includes a data-transmission channel having two pairs of lines. However, this method is not readily usable for detecting interfaces having a different number of line pairs.